In recent years, accompanying the adoption of multimedia information in cellular mobile communication systems, it has become common to transmit not only speech data but also a large amount of data such as still image data and moving image data. Furthermore, studies are being actively conducted in LTE-Advanced (Long Term Evolution Advanced) to realize high transmission rates by utilizing broad radio bands, Multiple-Input Multiple-Output (MIMO) transmission technology, and interference control technology.
In addition, taking into consideration the introduction of various devices as radio communication terminals in M2M (machine to machine) communication and the like as well as an increase in the number of multiplexing target terminals due to MIMO transmission technology, there is a concern regarding a shortage of resources in a mapping region for a PDCCH (Physical Downlink Control Channel) that is used for a control signal (that is, a “PDCCH region”). If a control signal (PDCCH) cannot be mapped due to such a resource shortage, downlink data cannot be assigned to the terminals. Therefore, even if a resource region in which downlink data is to be mapped (i.e., a “PDSCH (Physical Downlink Shared Channel) region”) is available, the resource region may not be used, which causes a decrease in the system throughput.
As a method for solving such a resource shortage, a study is being made of assigning, in a data region, control signals for terminals served by a radio base station apparatus (hereunder, abbreviated as “base station”). A resource region in which control signals for terminals served by the base station are mapped is referred to as an Enhanced PDCCH (ePDCCH) region, a New-PDCCH (N-PDCCH) region, an X-PDCCH region or the like. Mapping the control signal (i.e., ePDCCH) in a data region as described above enables transmission power control on control signals transmitted to a terminal near a cell edge or interference control for interference by a control signal to another cell or interference from another cell to the cell provided by the base station.
Further, according to the LTE-Advanced system, to expand the coverage area of each base station, relay technology is being studied in which a radio communication relay station apparatus (hereunder, abbreviated as “relay station”) is installed between a base station and radio communication terminal apparatuses (hereunder, abbreviated as “terminals”; may also be referred to as UE (user equipment)), and communication between the base station and terminals is performed via the relay station. The use of relay technology allows a terminal that cannot communicate with the base station directly to communicate with the base station via the relay station. According to the relay technology that has been introduced in the LTE-Advanced system, control signals for relay are assigned in a data region. Since it is expected that the control signals for relay may be extended for use as control signals for terminals, a resource region in which control signals for relay are mapped is also referred to as an “R-PDCCH.”
In the LTE (Long Term Evolution) system, a DL grant (also referred to as “DL assignment”), which indicates a downlink (DL) data assignment, and a UL grant, which indicates an uplink (UL) data assignment are transmitted through a PDCCH. The DL grant indicates to the terminal that a resource in the subframe in which the DL grant is transmitted has been allocated to the terminal. In an FDD system, the UL grant indicates that a resource in a target subframe that is the fourth subframe after the subframe in which the UL grant is transmitted has been allocated to the terminal. In a TDD system, the UL grant indicates that the resource in a target subframe that is the fourth or a subframe subsequent to the fourth subframe after the subframe in which the UL grant is transmitted has been allocated to the terminal. In the TDD system, which one of subframes located after the subframe in which the UL grant is transmitted is to be taken as the target subframe to be assigned to the terminal depends on the time-division pattern of the uplink and downlink (hereinafter referred to as “UL/DL configuration pattern”). However, in every UL/DL configuration pattern, the UL subframe is the fourth subframe after the subframe in which the UL grant is transmitted or a subframe subsequent to the fourth subframe.
In the LTE-Advanced system, a region (R-PDCCH for relay station (relay PDCCH) region) in which channel control signals for relay stations are mapped is provided in the data region. Similarly to the PDCCH, a DL grant and a UL grant are mapped to the R-PDCCH. In the R-PDCCH, the DL grant is mapped in the first slot and the UL grant is mapped in the second slot (refer to Non-Patent Literature “hereunder abbreviated as NPL” 1). Mapping the DL grant only in the first slot reduces a delay in decoding the DL grant, and allows relay stations to prepare for ACK/NACK transmission for DL data (transmitted in the fourth subframe following reception of the DL grant in FDD). Thus, each relay station monitors channel control signals transmitted using an R-PDCCH from a base station within a resource region indicated by higher layer signaling from the base station (i.e., a “search space”) and thereby finds the channel control signal intended for the corresponding relay station.
In this case, the base station indicates the search space corresponding to the R-PDCCH to the relay station by higher layer signaling.
In the LTE and LTE-Advanced systems, one RB (resource block) has 12 subcarriers in the frequency domain and has a width of 0.5 msec in the time domain. A unit in which two RBs are combined in the time domain is referred to as an RB pair (for example, see FIG. 1). That is, an RB pair has 12 subcarriers in the frequency domain, and has a width of 1 msec in the time domain. When an RB pair represents a group of 12 subcarriers on the frequency axis, the RB pair may be referred to as simply “RB.” In addition, in a physical layer, an RB pair is also referred to as a PRB pair (physical RB pair). A resource element (RE) is a unit defined by a single subcarrier and a single OFDM symbol (see FIG. 1).
The number of OFDM symbols per RB pair changes depending on the CP (cyclic prefix) length of the OFDM symbols. Further, the number of REs of a resource region in which an ePDCCH is mapped per RB pair differs depending on the number of OFDM symbols and the number of REs used for a reference signal (RS).
The number of OFDM symbols and a reference signal that can be used vary for each subframe. Accordingly, in a subframe having a small number of REs of a resource region in which an ePDCCH is mapped in a single RB pair, the ePDCCH reception quality decreases.
Further, the number of OFDM symbols used for a PDCCH is variable between one and four. Accordingly, in a case where a PDCCH region is not configured for an ePDCCH, the number of OFDM symbols that can be used for an ePDCCH decreases as the number of OFDM symbols of the PDCCH region increases.
Further, the number of REs to be used for a reference signal differs according to the configuration of the reference signal as described below (see FIG. 1).
(1) CRS (1, 2, 4 Tx):
A CRS (cell specific reference signal) is transmitted in all RBs. Although a CRS may also be transmitted in a data region in a subframe other than an MBSFN subframe, in an MBSFN subframe, a CRS is transmitted using only the first two OFDM symbols. The position at which the CRS is mapped varies depending on the cell ID.
(2) DMRS (12 REs, 24 REs or 16 REs):
Utilization of a DMRS (demodulation reference signal) is dynamically indicated to the terminal from the base station by downlink assignment control information (DL assignment). The number of DMRSs that are configured can be varied for each user. The DMRS is transmitted in a data region.
(3) CSI-RS (2 REs or More):
A CSI-RS (channel state information reference signal) is transmitted in all RBs. The subframe to be transmitted depends on a predetermined period.
A PDCCH and an R-PDCCH have four aggregation levels, i.e., levels 1, 2, 4, and 8 (for example, see NPL 1). Levels 1, 2, 4, and 8 have six, six, two, and two “mapping candidates,” respectively. As used herein, the term “mapping candidate” refers to a candidate region to which a control signal is to be mapped, and a search space is formed by a plurality of mapping candidates. When a single aggregation level is configured for a single terminal, a control signal is actually mapped to one of the plurality of mapping candidates of the aggregation level. FIG. 2 illustrates an example of search spaces corresponding to an R-PDCCH. The ovals represent search spaces for each of the aggregation levels. The multiple mapping candidates in each search space for each aggregation level are located in a consecutive manner on VRBs (virtual resource blocks). The resource region candidates in the VRBs are mapped to PRBs (physical resource blocks) through higher layer signaling.
Studies are being conducted with respect to individually configuring search spaces corresponding to ePDCCHs for terminals. Further, with respect to the design of ePDCCHs, part of the design of the R-PDCCH described above can be used, and a design that is completely different from the R-PDCCH design can also be adopted. In fact, studies are also being conducted with regard to making the design of ePDCCHs and the design of R-PDCCHs different from each other.
As described above, a DL grant is mapped to the first slot and a UL grant is mapped to the second slot in an R-PDCCH region. That is, a resource to which the DL grant is mapped and a resource to which the UL grant is mapped are divided on the time axis. In contrast, for ePDCCHs, studies are being conducted with regard to dividing resources to which DL grants are mapped and UL grants are mapped on the frequency axis (that is, subcarriers or PRB pairs), and with regard to dividing REs within an RB pair into a plurality of groups.